The present invention relates to a receiving apparatus for receiving signals in a digital telecommunication system and to a synchronising method for synchronising such a receiving apparatus. Particularly, the receiving apparatus and the synchronising method of the present invention use a cross correlation mechanism to achieve accurate time and frequency synchronisation.
Digital telecommunication systems generally need a synchronisation of a transmitting side and a receiving side. The transmitting side and the receiving side can e.g. be base stations and mobile stations of a telecommunication system, whereby the synchronisation of the timing and the frequency of transmitted signals is usually performed in the mobile station. To achieve a synchronisation, it is known to transmit a special training sequence or a reference symbol. Such a reference symbol is usually embedded in the transmission data structure and regularly sent so that a synchronisation can be performed regularly.
In FIG. 1, a general structure of a receiving apparatus is shown. The receiving apparatus can e.g. be a mobile station of a wireless digital telecommunication system. Although the present invention essentially relates to the receiving part of a telecommunication terminal, it is to be understood, that the receiving part or receiving apparatus of the present invention can also be a or part of a receiving and transmitting terminal.
The receiving apparatus 1 shown in FIG. 1 comprises an antenna 2 for receiving signals from a transmitting side, e.g. a base station of a wireless digital telecommunication system. The received signals 2 are supplied to a HF means (High Frequency means) 3, which downconverts the received high frequency signals into the base band. The downconverted signals are supplied to a IQ-demodulation means, where they are demodulated and supplied to a synchronising means 5.
The synchronising means performs time and frequency synchronisation using a received training sequence or reference symbol, as stated above. Using the synchronisation information of the synchronising means 5, the received user data signals are further processed in the receiving apparatus 1, e.g. decoded by a decoding means 6 and so on, to be made available in visible or audible form for a user. Usually the synchronisation in the synchronising means 5 is performed in the time domain.
Generally speaking, the synchronising means 5 performs a time domain correlation between the reference symbol (or parts of the reference symbol) and a delayed version of the received reference symbol (or parts of the reference symbol) to identify the reference symbol (or parts of the reference symbol) and thus to determine the timing for the synchronisation. Thereby, a correlation peak is calculated, which should correspond as accurate as possible to the time point of the last sample of the reference symbol.
In order to achieve a well detectable correlation peak, the reference symbol usually consists of a plurality of synchronisation patterns, which are repeated several times within one reference symbol period. The synchronisation patterns usually have the same shape or form and are thus called repetition patterns throughout the present application. A reference symbol therefore contains several repetition patterns, whereby each repetition pattern consists of a plurality of samples. Each repetition pattern has the same number of samples. Between the reference symbol and the adjacent user data symbols, guard intervals can be inserted to avoid intersymbol interference in a multipath environment of the telecommunication system.
The time domain correlation of the received reference symbol in the receiving apparatus 1 can be achieved e.g. on the basis of an auto correlation mechanism or a cross correlation mechanism. An auto correlation mechanism thereby does not require any knowledge about the reference symbol on the receiver side, whereby a cross correlation mechanism requires exact knowledge about the reference symbol to be received on the receiver side. As stated above, the present invention particularly relates to a receiving apparatus and a synchronising method which use a cross correlation mechanism.
A known cross correlation means 7 is shown in FIG. 2. The cross correlation means 7 cross correlates incoming signals y(i), e.g. coming from the IQ demodulation means 4, within a cross correlation window of a length 16. The cross correlation window length 16 means that the incoming digital signal y(i) is cross correlated sample by sample on the basis of a length of 16 samples. The cross correlation window length of 16 samples can thereby correspond to the length of a repetition pattern of the reference symbol. In FIG. 3, a reference symbol comprising 9 repetition patterns is shown, whereby one repetition pattern can comprise 16 samples. The receiving apparatus 1 knows exactly the structure of the reference symbol to be received. A complex conjugated version of an expected repetition pattern is stored in the synchronising means 5 and cross correlated to the received signals.
The cross correlation means 7 of FIG. 2, which has a cross correlation window length of 16, comprises 15 delay means 8 arranged serially. The first delay means delays the incoming complex signal y(i) by one sample, which corresponds to multiplication with a factor z−1. The second delay means delays the output of the first delay means again by 1 sample and so on. Further, the cross correlation means 7 comprises 16 multiplication means 9 and a sum means 10. The delay means 8, the multiplication means 9 and the sum means 10 are arranged so that an incoming signal having a length of 16 samples is cross correlated with a complex conjugated version of the samples of a repetition pattern. The complex conjugated samples of the expected repetition pattern are e.g. stored in the synchronising means of the receiver and read out respectively to the multiplication means 9. E. g. a first received sample y(0) is multiplied with a complex conjugated version of the first sample of the expected repetition pattern, i.e. y*(0)=s0*. The next received sample y(1) is multiplied with y*(1)=s1*, and so forth. The sum means 10 adds up all the results from the multiplication means 9, so that an output signal r(i) is obtained. The output signal r(i) of the sum means 10 is supplied to an absolute value calculating means 11 which calculates the absolute value of r(i) to detect a cross correlation peak. The cross correlation means 7 and the absolute value calculating means 11 shown in FIG. 2 can be comprised in the synchronising means 5 of the receiving apparatus 1 shown in FIG. 1.
In FIG. 3, the cross correlation peak detection performed by the cross correlation means 7 and the absolute value calculating means 11 shown in FIG. 2 is explained. FIG. 3 shows three different phases of a cross correlation calculation of an incoming signal. In phase 1, the correlation window 13 of the cross correlation means 7 is located on received user data, which means that only user data are cross correlated. The user data are indicated by “??? . . . ”. Thus, no cross correlation peak is detected. In phase 2, the correlation window 13 is exactly matching with the eighth repetition pattern S7 of the reference symbol 12, so that a corresponding cross correlation peak is detected. In phase 3, the cross correlation window 13 is again cross correlating user data “??? . . . ”, so that no cross correlation peak is detected.
The reference symbol 12 shown in FIG. 3 comprises 9 repetition patterns S0, S1, . . . , S8, which have identical shapes. Each of the repetition patterns comprises e.g. 16 samples, which corresponds to the cross correlation window length 16 of the cross correlation means 7 in FIG. 2. Of course, the number of repetition patterns in the reference symbol 12 and the number of samples in each repetition pattern can be changed and adopted to the respective application.
As stated above, the cross correlation mechanism requires exact knowledge on the reference symbol to be received on the receiving side. This means, that the receiving apparatus needs to know exactly the structure and number of repetition patterns to be able to recognise the last cross correlation peak, which serves for a time and frequency synchronisation. On the other hand, if one of the cross correlation peaks is not properly detected, the synchronisation fails. In mobile communication environments, in which multipath fading degrades the correlation peak detection performance, the synchronisation performance in a known receiving apparatus of the telecommunication system is thus significantly lowered.